Axial length of relief



N. M. PACKARD ET AL Re. 26,764

Jan. 13, 1970 PISTON 3 Sheets-Sheet 1 Original Filed Sept. 15, 1965lax/2591's (Zamza re Wfac fa/ac! I500 was [/2]! 67a}! XIAL LENGTH OFRELIEF IOOOHBS.

Jan 13, 1970 N M PACKARD ET AL Re. 26,764

PISTON Original Filed Sept. 13, 1965 3 Sheets-Sheet 2 aw Z6 P'CnmmWPaavZa c flyw fame I 34 ww.

@arcsy Jan. 13, 1970 N. M. PACKARD ET AL Re. 26,764

PISTON Original Filed Sept. 13, 1965 3 Sheets-Sheet 3 1 qraa 1 4 34 jJ-cxZe/mb 56 1 i ?Zazm m YZPQQM @0035 United States Patent 26,764 PISTONNorman M. Packard, Des Plaines, Ill., and Howard R. Cook, Santurce,Puerto Rico, assignors to International Harvester Company, a corporationof Delaware Original No. 3,357,318, dated Dec. 12, 1967, Ser. No.490,154, Sept. 13, 1965. Application for reissue Sept. 6, 1968, Ser. No.771,682

Int. Cl. F16j 1/14; B23p /10 U.S. Cl. 92-187 6 Claims Matter enclosed inheavy brackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE Piston having a wrist pin bore which issurface hardened, the normally cylindrical surface whereof beingrelieved in areas but unrelieved elsewhere to retain cylindricalness andthe hardness. Chances of cracking the piston are reduced because theareas of relief are at points in the bore walls intermediate the ends fthe bore at places on diametrically opposite sides thereof. Theresulting relief accommodates widthwise elliptical expansion of atubular wrist pin which when provided in the bore tends to flatten, andthe bore presents unrelieved, hard cylindrical areas in the directcompression path for withstanding the loading.

This invention relates to a piston for engines, and particularly to apartially relieved connection between the wrist pin and the pin bossesin the piston. Each piston is carried at the end of a so-calledconnecting rod, and the Wrist pin forms a pivot joint connecting thetwo.

We preferably provide pin bore relief to accomplish the purpose of theinvention, and desirably the intc.fitting wrist pin is of a tubularconstruction, circular in cross section. The selection of where toprovide the relief between the coextensive surface of each boss with thewrist pin, and where not to provide relief is based upon carefulconsideration of the piston problem in an engine, particularly in arelatively high speed, direct injection diesel engine. The compressionloads and side thrust loads due to combustion pressures are rathersevere.

The immediate problem of concern herein is piston cracking, which occursat the two points of obvious maximum compressive stress in the wrist pinbore as might be expected, but which is due to a distorting forceoriginating in an altogether different manner and at a separate locationin the piston. Many factors are involved in getting at this problem.

The tubular wrist pin constitutes a comparatively small reciprocatingmass and a strong, heavy metal is used, which is steel in just about allcases. The piston is a large member and is made of aluminum so as tokeep down the overall reciprocating mass and to allow a less stressed,lighter crankshaft and lower bearing loads to be employed. The highercoefficient of thermal expansion of aluminum compared to steel can beseen to manifest itself as a difficulty when the three conditions of thesteel pin and aluminum piston are considered. The Ambient condition ofassembly of the piston and pin is usually room temperature andpreferably the pin enters the bore under low pressure or even handpressure. The maximum diameter of the pin exceeds the bore diameter atmost by only 0.0001 inch so as not to interfere with assembly.

In the transient condition beginning at ambient temperature and lastingthrough cranking and through warm up of the engine to its hot runningcondition, clearance may or may not develop in the connection betweenthe wrist pin and the bored pin bosses, depending upon the "ice relativerate of warm up of the aluminum metal and the steel metal. In any case,when the piston is subjected to heavy gas pressure loading in thedirection of piston movement during warm up, the head load istransmitted between the surface of each boss and the wrist pin on theside of the pin adjacent the head, and is localized precisely at theinner edge of each boss. The period of this transient condition is themost critical period even though a transient one, because of thetendency of the wrist pin to compress or flatten so that the higheststressed portion expands outwardly in a direction perpendicular to thedirection of movement of the piston, and possibly before the wrist pinbore has reached its normal maximum dimension attained in the hotrunning condition of the engine.

In the hot running condition, the wrist pin bore will have expanded toapproximately 0.002 inch greater than comparative points of the diameterof the wrist pin, due to the differences in the coefficients of thermalexpansion of the two metals referred to. This opening up of an extremelyliberal running clearance is not to be desired, but can be readilycompensated for as will be explained.

The gas loads within the combustion chamber of the engine cause a sidethrust to be transmitted between the surface of each boss and the wristpin on a skirt side of the piston, and impose a head load tending toincrease the area of contact between each boss and the wrist pin on thehead side. In a piston properly designed according to our invention, theside thrust is transmitted between the outer ends of the wrist pinbosses and the corresponding ends of the wrist pin. The reason is thatthe maximum flattening or ellipticalness of the wrist pin alwayslocalizes at the inner end of the bosses, which must be oversircd byrelief at the right points in the pin bore to avoid the problemdescribed in connection with warm up. The ellipticalness is used toadvantage at the point of maximum compressive stress because of theincrease in area of contact where needed.

As viewed at points along its length simultaneously, the piston wristpin varies in cross section from ellipticalness at the point of maximumcompressive stress to substantial circularity at the ends of the pin. Asso viewed at that time, the pin ends bend slightly in the direction ofpiston movement during maximum gas pressure conditions, but the pinrestraightens between its ends when unstressed during no-load conditionin its movement.

Without the relief aforementioned, the outwardly expanding ellipticalsection of the wrist pin which occurs perpendicular to the direction ofpiston movement would create tension stress at the point of maximumcompression on the head side of the wrist pin. Cracking would start ineach boss and progress in the direction toward the head; in anexaggerated case of no relief, the two cracks would join and eventuallydivide the piston into two pieces.

Although there are satisfactory other ways of seeing how to produce therelief, one easy way of visualizing it is to consider a preferred methodof piston manufacture provided by our invention. The preferred method,practiced in treating the surface of a wrist pin bore in a piston havinga head, having a skirt with the bore cored therein, and having bossesthrough which the bore passes and which integrally join and reinforcethe head and skirt, comprises the steps of work hardening the entirebore before or after machining the bore to a finished surface; bevelingthe bore on both of the bore sides adjacent the piston skirt bypositioning tandem rotary cutters, having equal radii smaller than thepin bore. eccentric to the longitudinal axis of the pin bore at the twoplaces on opposite sides of said axis and in a direction along thattransverse axis of the bore which is perpendicular to the normallongitudinal movement of the piston, the cutters being tapered so as tohave an increasing diameter in the inward direction; and relieving thebore so that the bevel is maximum at the inner edge of each boss andmerges with the bore surface along an arc of intersection at the outerend of the relief falling at no more than approximately halfway to thecorresponding outer end of the bore.

According to one disclosed modification of bore treatment hereof havingadvantages that will become apparent hereinafter, the bore in the pistonis machined moderately undersized, the metal in the bore is surfacehardened by work hardening (and consequently slightly radiallyenlarging) the bare to a condition of being slightly undersized, and thebore is then finish machined a slight further amount to final bore size.According to another disclosed modification of treatment, the bore ismachined slightly undersized, and the metal is then surface hardened bywork hardening (and consequently slightly radially enlarging) the boreto final bore size. Following treatment according to either modificationdisclosed, the bore at points intermediate its ends is then processed asdescribed by relieving the walls at the places on both sides adjacentthe piston skirt.

The maximum work hardening necessary in the bore goes only to a minordepth in the metal of the pin bosses to satisfactorily handle thecompressive stresses therein. We have discovered that work hardening, toan extent substantially in excess of the hardening [necessary for]normally from compressive stresses, is desirable if not essential inalleviating the piston cracking problem. In other words, selected pinbore relief in conjunction with substantial work hardening of the borematerially reduces piston cracking, whereas relief alone or workhardening alone has only minor effectiveness in that regard.

Further features, objects, and advantages of the invention will bespecifically pointed out or become apparent when for a betterunderstanding of the invention, reference is made to the followingwritten description taken in conjunction with the accompanying drawingswhich show a preferred embodiment thereof, and in which:

FIGURE 1 is a rear elevational view of a piston and connecting rodhaving a connection embodying the present invention;

FIGURE 2 is a top elevational view of the piston appearing in FIGURE 1;

FIGURES 3 and 4 are respectively a longitudinal side elevational viewand a bottom plan view taken along the section lines 33 and 44 of FIGURE1;

FIGURE 5 is an isometric view schematically illustrating the runningclearance and relieved clearance between the opposing metal surfaces ofthe wrist pin and pin bosses of the piston;

FIGURES 6 and 7 are transverse end elevational views of a detail takenalong the section lines 6-6 and 7-7 of FIGURE 4;

FIGURE 8 is a graph showing the life tendencies of pistons havingdiffering amounts of the present pin bore relief:

FIGURE 9 shows a tool work hardening the wrist pin bore in the piston:and

FIGURE 10 shows the actual relieving of the pin bore following workhardening, and yielding the piston construction shown in FIGURE 4.

More particularly in FIGURES l and 2 of the drawings. a piston of anengine is shown having a gasloaded face 22 at the head. A slightlyoffset cavity 24 in the head functions as a squish chamber at the upperend of the compression stroke of the piston 20 within a cylinder, notshown. Shallow circular recesses 26 and 28 which intersect the squishcavity 24 provide relief in the surface 22 to clear the valves as thepiston 20 reaches the upper end of its exhaust stroke.

A transversely disposed wrist pin bore 30 intersects the skirt 32 of thepiston at the outer ends of the bore and is unoccupied at the outerends. inwardly from each outer end, a snap ring groove 34 is formed inthe bore and the grooves carry two snap rings 36 for retaining, betweenthe rings, a hollow steel wrist pin 38. A connection rod 40 is connectedat its piston end 42 to the piston 20 by means of the wrist pin 38, andis connected at the opposite end by means, not shown, to the throw of acrankshaft 44. The direction of rotation of crankshaft, as viewed fromthe rear in FIGURE 1, is counterclockwise as shown by an arrow 46.

Gas pressure exerted as shown by arrows 48 upon the piston head surface22 causes a reactive force in the rod 40 indicated by a force vector 50,which vector is conveniently resolved into an upwardly acting componentindicated by an arrow 52 and a side thrust component indicated by anarrow 54. A set of piston rings 56 received in grooves 58 at or adjacentthe head of the piston 20 cooperates with the cylinder walls, not shown,to prevent the escape of gas from combustion.

In FIGURES 3 and 4, two wrist pin bosses 60 and 62 integrally jointogether the head of the piston 20 and the skirt 32, reinforcing thetwo. The pin bore 30 is seen to be relieved on the skirt sides. Morespecifically, it has two semi-circular relieved areas 64 and 66 adjacentthe end of the boss 60, and two semi-circular relieved areas 68 and 70adjacent the inner end of the boss 62.

When the parts of the engine reach a temperature equivalent to ahot-running condition, the aluminum bore in the piston slightly exceedsthe wrist pin diameter. The resulting fit between the pin and boreprovides indeed a liberal, if not overly liberal, amount of runningclearance on the bottom or inertia side of the wrist pin, at a time whenthe weight of the piston in a static, unstressed condition, establishesa narrow area or a line of contact at the top or head side of the wristpin due to the difference in diameters between the pin and the bore.

The clearance relationship just referred to its schematicallyillustrated in FIGURE 5. The skirt-side areas 68 and 70 in the walls ofthe bore 30 form Wedge-like spaces of relative relief which are far inexcess of running clearance. The running clearance on the inertia sideof the wrist pin 38, though liberal in amount as indicated, is greatlyexaggerated for illustration purposes in FIGURE 5. The relief space atthe sides of the pin and bore is seen to disappear in crescent-likeconverging arcs in the bosses adjacent the piston head and, at least intheory, line contact results due to the bore diameter being in excess ofthe pin diameter by approximately 0.002 inch.

The most greatly deformed portion of the wrist pin resulting fromcombustion pressure during the hot running condition is illustrated inFIGURE 6. Each of the relieved areas 68 and 70 is preferably symmetricalto itself about a horizontal plane and each has an arcuate extent ofapproximately Consequently, the wrist pin 38 is able to elastically mashinto an elliptical shape from top to bottom, thus conforming to thelarger arc of, and being coextensive with, the hardened cylindricalsurface of [and] an unrelieved portion 71 of the bore constituting theupper 70 of that bore. Thus, the vertical component 52 of the rodreaction vector, not shown, is not concentrated in a theoretical linecontact, but is distributed over an entire arcuate area adjacent theinner end of each of the wrist pin bosses 60 and 62. The major dimensionbetween the relief areas 68 and 70. i.e., the dimension perpendicular tothe direction of piston movement, exceeds the corresponding majordimension of the wrist pin 38 when flattened, both during the hotrunning condition and the warm-up condition of the engine so as toinsure clearance during the latter condition.

The bosses 60 and 62 define a connecting rod slot or cavity 74 so thatthe piston end 42 (FIGURE 1) of the connecting rod has clearance tooscillate about the wrist pin axis indicated at 76 (FIGURE 3). When gaspressure is maximum on the face 22 of the piston head, the downwardforce of the piston tends to concentrate in accordance with the arrowsshown at 78 in FIGURE 3. Those arrows are on the boss side of the cavity74 whereas the force of the connecting rod, not shown, upon the wristpin tends to be slightly inwardly offset so as to concentrate on the rodside of the cavity according to the arrows 80 shown in broken lines. Asone phase of distortion, the wrist pin, not shown, bends between itsends along an axis which is indicated at 82 in a somewhat exaggeratedfashion. The bending is due to the two force couples created by theoffset loading involved, namely, the forces in the direction of thearrows 80 and the opposing forces in the direction of the arrows 78.

In FIGURE 7, the relation of parts is shown under the circumstances ofgas pressure bearing down upon and having advanced the piston part waydown on its power stroke. Each outer end of the wrist pin 38 which isshown is practically circular in cross section but the pin is bent inits length-wise direction because of the force couples just described.The piston head side of the wrist pin is pressed in the direction of thepin boss 62, and the wrist pin on one side of the skirt sides, namely,on the thrust side of the skirt, reacts the horizontal component 54 ofthe connecting rod force vector directly into the unrelieved coextensivesurface of the pin boss 62.

Because of such nonrelief of the bore 30 at the two points involved, therespective pin bosses 60 and '62 provide solid metal backing to thewrist pin ends, as indicated by the arrows 72 thereto in FIGURE 4.

Tests made, though not comprehensive, give definite indications ofpreferred values for the axial length of the relief areas within eachspecific wrist pin bore. Pistons of a 4 /2 size were relieved to various[extends] extents in the bore and installed in engines for life testsunder conditions which tended to produce piston cracking. The engineswere direct injection, high speed diesel engines and they were run notat steady conditions but at periodically varied conditions of speed andload which seem to promote cracking.

The results are represented in graphic form in FIG- URE 8. The optimumaxial length of relief on the basis of the present results is Va" forthe 4%" pistons tested. Those pistons were run for 1500 hours under thecircumstances described, without a failure. On the other hand, aslightly longer length of relief in the axial direction and a slightlyshorter length of relief in the axial direction gave rise to earlierfailure of the pistons, some of which cracked without accumulating asmuch as 1000 hours of running. It is not known what complete testresults would show, but it appears to follow that the relief dimensionscan be quite critical and that, if the tapered, partial relief isextremely underdone or extremely overdone throughout the full availablelength of the contact between the bore and wrist pin, optimum liferesults will not be obtained.

In FIGURE 9, the performance of the work hardening operation of the bore30 is illustrated. A turning arbor 82 has a set of rotatably mountedrollers 84 around the periphery so as to roll against the bore 30 at oneend, and the arbor has another set of rotatably mounted rollers 86around the periphery so as to rotate against the bore 30 at the oppositeend. The metal is work hardened to a depth of several thousandths of aninch, or slightly more, due to the pressure of the rotating rollers asthey are revolved about the arbor axis. The contact pressures are notexcessive but they are sufficient to work harden the aluminum.

The foregoing operation can be performed with good results after thecored-in bore 30 has been finish machined. The work hardening is not toostrenuous, how ever, because of the possibility that the bore wouldbecome out of round or lose its finished dimension.

In case the wrist pin bore 30 is rough machined only, then the workhardening operation is more rigorous and extends for slightly more thanseveral thousandths inch in depth within the worked metal of the bosses60 and 62. Then the bore 30 is finish machined, the cutting or honingbeing only to a depth of one or two or three thousandths of an inch soas not to machine away the entire depth of work hardened metal.

In either case, the resulting pin boss bore structure for receiving thewrist pin comprises aluminum metal of which part is contiguous withhardened portions embracing the bore 30, the embracing portions beingconstituted of the bearing surface of the bore and metal of the bossesbacking up the surface of the bore, the embracing portions having beenhardened in the respective bosses by a work hardening tool or mechanismsuch as the rollers 86 (FIGURE 9) so that, in their radial extent asalready seen, the embracing portions go to at least a minor depth in thebore 30 and are harder than the contiguous metal of the bosses.

The relieving operation of the opposite sides of the inner sides of thebore is illustrated in FIGURE 10. Preferably, an arbor 86 carries a pairof tandem beveled cutters 88 and 90 each received in a different end ofthe bore 30. The adjacent ends of the cutters are the large ends and therelief progresses from the inner end of the bosses so as to taper towarda point of intersection with the unrelieved portion of the bore at theouter end. The arbor 86 rotates and, in the position shown in solidlines in FIGURE 10, pressure is applied so that the teeth on the cutters88 and 90 relieve the bore, forming the completed areas 64 and 68. Thearbor 86 is then moved crosswise into the dotted line position shown bydotted lines 86a and companion areas are relieved on the opposite sideof the bore 30. At this point all work of treating the bore 30 iscompleted.

The respective diameters of the cutters and the wrist pin bore verynearly equal one another. but the difference therebetween is greatlyexaggerated in FIGURE 10 to emphasize the effect. So also the bevel onthe tapered cutters is greatly exaggerated and the bevel of the relievedsurface areas is greatly exaggerated.

Following is an example of a more complete set of specifications inconnection with one piston referred to:

Piston diameter 4.500". Wrist pin bore diameter 1.625. Wrist pin borelength 4.500.

Representative angles of face of cutter (bevel) relative to wrist pinbore axis Representative angles of taper of relief Diameter of cutteralong portion which is first in contact In the piston as hereindisclosed, a floating wrist pin is used, having running clearance withrespect to both the rod and the pin bosses so as to be free to rotatetherein. The wrist pin is generally circular in cross section and,irrespective of its rotated position, the pin always tends to flatten onthe extreme piston head side and on the extreme inertia load sidethereof, i.e., it flattens perpendicular to the direction of pistonmotion.

Consistent with the foregoing, the hardened surface of the borepresented by unrelieved portions such as the portion 71 (FIGURE 6) whichare loaded in the direct path of longitudinal compression of the piston20 has particular benefit. The present improvement in alleviating pistoncracking is believed to reside in the provision and retention of surfacehardening in critical ones of the unre- Iieved areas.

The relieved areas in the bore, consistent with the foregoing, are atall times perpendicular to the direction of travel of the piston becauseneither the relieved areas of the bore nor the piston have anappreciable rocking motion. The present improvement in alleviatingpiston cracking is believed to reside in the location and shape of theparticular two wedge-like spaces which result, and which provide no fulllength taper throughout the bore. Instead, the relieved bore areas mergeWith the bore surface along a semi-circular arc of intersection(technically, a parabola) at the outer end of the relief falling at nomore than approximately halfway to the corresponding outer end of thebore. The best results actually observed in some incomplete testingoccurred when the relieved surface extending from each inner or boss endof the bore toward the outer end Was 43% of the distance from the bossend to the outer end of the bore.

What is claimed is:

1. In a piston,

a head having a gas-loaded face;

a skirt;

spaced-apart bored pin bosses joined between the head and skirt, saidbored pin bosses being comprised of one material, of which the entireinner periphery of the bore is hardened relative to the surrounding n1nterinl to at least a minor depth radially of said bore; and

a connection between a hollow wrist pin and the bored pin bossesproviding clearance in the bore, said pin having skirt sides spaced 90degrees from both the side of the pin adjacent the piston head and theside 0) the pin opposite the piston head;

[a connection between a hollow wrist pin and the bored pin bossesproviding clearance in the bore,]

the coextensive surface of each boss with the pin having no more thanrunning clearance space for relative oscillatory motion, except forwedge-like spaces of relative relief in the surface which are segmentsof a circle and are confined to both skirt sides of the pin, eachwedge-like space being thick at the inner edge of the boss and generallydecreasing in thickness outwardly therefrom, whereby gas loads Y on saidface which flatten the wrist pin and decrease the thickness of eachwedge-like space, will cause a side thrust to be transmitted between theunrelicved surface of each boss and the wrist pin on a skirt side of thepin, and impose a head load transmitted between the surface of each bossand the wrist pin on the side of the pin adjacent the head, in a mannerto increase the area of contact between each boss and the wrist pin onsaid head side.

2, The invention of claim 1,

the inertia load being transmitted between the surface of each boss andthe wrist pin on the side of the pin opposite the piston head.

3. The invention of claim 1,

said wedge-like spaces being formed by the metal of the bored bossesbeing relieved to provide same, the

depth of relief comprising a bevel in the bore which increases uniformlyin thickness inwardly with respect to the bore, the thickness beingmaximum at the inner edge of each boss and merging with the bore surfacealong an arc of intersection at the outer end of the relief falling atno more than approximately halfway to the corresponding outer end of thebore.

4. The invention of claim 3,

said wrist pin being made of steel, the piston being made of aluminum.

5. In a piston for a reciprocating engine having a piston head, a skirtdepending from the head, and bosses cast integrally with and connectingthe head and skirt, [the improvement comprising:]

[a bore in the piston] improved pin boss bore structure in the pistonformed with a bore in the bosses extending therethrough and having skirtsides of the bore spaced 90 degrees from both the side of the boreadjacent the piston head and the side of the bore opposite the pistonhead, the improvement in said structure specifically charized whereby:

said pin bosses being comprised of one material of which the entireinner periphery of the bore is hardened relative to the surroundingmaterial to at least a minor depth radially of said bore;

said bore in the piston adapted for receiving a wrist pin with no morethan running clearance space for relative oscillatory motion, except fora part length taper formed in the bore relatively relieving the surfaceso as to form semicircular areas which are confined to both skirt sidesof. the bore, the maximum thickness of relief occurring at the inneredge of each boss in a radial direction to the latter and perpendicularto the direction of piston movement, said tapered relief being ofuniformly decreasing thickness outwardly from said maximum thickness,said tapered relief merging with the bore surface along an arc ofintersection at the outer end of the relief falling at no more thanapproximately halfway to the corresponding outer end of the bore.

6. The invention of claim 5, wherein the angle of taper of said reliefis at least approximately 2 with respect to the wrist pin bore axis.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 1,923,717 8/1933 Frelin 184-6 2,387,634 10/1945Anderson 92-187 2,388,363 11/1945 Moore 92-l87 2,483,674 10/1949 Rosenet a1. 92-187 X 2,990,226 6/1961 Fangman 92-187 2,043,013 6/1937Shoemaker 92-187 2,437,824 3/1948 Kishline 29-1565 3,027,207 3/1962Meurer 92-187 3,123,899 3/1964 Townhill 29-1565 CARROLL B. DORITY, JR.,Primary Examiner U.S. C1. X.R. 29-1565

